Method for determining thickness, degree of cure and other properties of a polymeric coating or film

ABSTRACT

A method for monitoring a property of a polymeric mass such as a coating or film comprising the steps of adding a fluorescence probe compound capable of fluorescing to a polymeric composition which has the ability to undergo changes in microviscosity, the ratio of the intensity of the fluorescence emission of said compound at two wavelengths changing in response to said changes in said microviscosity of said composition; curing said composition; causing said compound to fluoresce; measuring the fluorescence of said compound; calculating the ratio of the intensities of fluorescence emission of said compound at two or more wavelengths; relating said ratio to the monitored property of said composition.

This is a continuation-in-part of application Ser. No. 08/238,459, filedMay 5, 1994.

BACKGROUND OF THE INVENTION

The present invention relates to a method for monitoring thickness,degree of cure, and other properties of a coating or film usingfluorescence methodologies.

Fluorescence spectroscopy has received considerable interest as ananalytical tool due to its high sensitivity, selectivity andnon-destructive characteristics. In addition, remote sensing is readilyavailable using fluorescence methods through the use of fiber-opticcables to transmit optical signals to and from the analytical site inreal time. Fluorescence spectroscopy has proven to be an extremelyuseful analytical tool in polymer chemistry. The technique has providedvaluable information on the mechanisms and kinetics of polymerization,curing and crosslinking as well as oxidation, degradation andstabilization. The technique of fluorescence spectroscopy has beenparticularly useful in elucidating polymer properties such as theirmacro or supermolecular structure, molecular weight, viscosity andpermeability.

Fluorescence probes as hereafter defined, have been used to studypolymerization kinetics. For example, Paczkowski and Neckers, "FollowingPolymerization Kinetics of Multifunctional Acrylates in Real Time byFluorescence Probe Methodology, Macromolecules, 25, 548-553 (1992)discusses the use of fluorescing probes such as dansylamide to followthe kinetics of polymerization and post-irradiation processes ofmultifunctional acrylates in real time. Other articles describe the useof fluorescence probes in polymerization studies of acrylates such asPaczkowski and Neckers, "Developing Fluorescence Probe Technology forMonitoring the Photochemical Polymerization of Polyolacrylates,"Chemtracts-Molecular Chemistry, vol. 3, 75-94 (1992); Paczkowski andNeckers, "New Fluorescence Probes for Monitoring the Kinetics ofLaser-Initiated Polymerization," JPS: Part A: Polymer Chemistry, vol.31, 841-846 (1993); and Zhang, Kotchetov, Paczkowski and Neckers, "RealTime Monitoring of Polymerization Rates of Polyacrylates by FluorescenceProbes II. Effect of Depth of Polymerization for a BleachingPhotoinitiator System," The Society for Imaging Science and Technology,vol. 36, No. 4, 322-327, July/August 1992.

DEFINITION

The term "fluorescence probe" as used herein means a compound, thefluorescence characteristics of which, when added to a fluid solutionwhich undergoes a change in microviscosity sometimes leading to a gel ora solid, are dependent upon the microenvironment of the fluorescingspecies. In accordance with the present invention, the changes in thefluorescent characteristics are related to changes in chemical,physical, and/or mechanical characteristics of the coating.

The term "cure" as used herein includes any process by which a polymericmass is hardened including, by way of example, crosslinking,polymerization, and solidification through evaporation of a solvent orthrough cooling (e.g., a hot melt composition).

SUMMARY OF THE INVENTION

In accordance with the present invention, methods are provided formeasuring the properties of an uncured, curing or a cured polymeric masssuch as a coating or film using fluorescence methodologies. Propertieswhich can be monitored in accordance with the invention include degreeof cure, hardness, abrasion resistance, resistance to strain, tactility,elasticity, molecular orientation, tensile strength, modulus, stressstate of solids, wetness and dryness. In one method, fluorescence isinduced in a substrate such that the thickness (or coat weight) of acoating on the substrate can be determined through an application ofBeer's law. The reduction in intensity due to absorption of substratefluorescence by the coating is used to monitor the thickness of thecoating. In another method, a fluorescence probe is provided in thecoating or film and the changes in the fluorescent characteristics ofthe probe are observed. Using previously prepared calibration curves,these changes can be related to various properties of the coating andparticularly to the degree of cure. While the invention is principallyapplicable to monitoring the properties of coatings and films, thoseskilled in the art will appreciate that these properties can be measuredin a polymeric mass of any geometry or shape provided, of course, thatthe mass in at least one dimension transmits the fluorescence such thatit can be detected. Hence, the invention broadly relates to the use ofthe disclosed methodologies in monitoring the properties of a polymericmass of any shape.

Using the foregoing fluorescence based methodologies, methods areprovided whereby the properties of a coating can be monitored in anindustrial setting. These methods may be used in-line or off-line toenable the coating manufacturer to determine any variance in the coatingand to adjust the coating parameters as necessary to correct for thevariance. Among the coating parameters that might be adjusted inresponse to a detected variance are coating application rate, linespeed, cure time, drying time, irradiation intensity, dosage, etc.

The present invention also provides an apparatus for monitoring coatingor film parameters in-line or near-line which comprises an array ofoptical fibers located adjacent the coating in a light-tight chamber ona coating manufacturing line which is coupled with a fluorescencedetector. A source of monochromatic excitation energy is associated withthe array such that fluorescence is induced in the substrate forthickness determination in accordance with the first method or in thecoating for determination of other properties in accordance with thesecond method. By sequentially sampling the fluorescence emission dataand comparing the observed fluorescence with calibration data using adata processor or computer, the coating thickness and properties can bemonitored on-line or near-line.

In more detail, one manifestation of the invention is a method formeasuring the thickness or coat weight of a polymeric coating on asubstrate. This method comprises the steps of:

providing a substrate having a nascent (inherent) fluorescence emissionor a substrate which has been doped with a fluorescer;

causing a first fluorescence intensity in said substrate at a firstpredetermined wavelength,

measuring said first fluorescence intensity;

applying a coating containing an absorber of said first fluorescenceonto said substrate;

causing a second fluorescence intensity at said first predeterminedwavelength;

measuring said second fluorescence intensity; and

determining the thickness of said coating based upon the ratio of saidfirst and said second fluorescence intensities.

While it is known in the art from Beers law that the thickness of acoating can be determined from the change in intensity of light passingthrough the coating and, more particularly, from the log of the ratio ofintensity I/Io where I is the intensity of light passing through thecoating and Io is the intensity of light incident the coating, thisproperty has not been applicable in many coating operations such as incoating containers due to the inability to position a detector on theside of the coating opposite the light source due to the presence of orinterference from the coated substrate. However, by using fluorescenceemission to measure thickness in accordance with the present invention,a light source in the form of fluorescence emission can be induced onthe side of the coating facing on the substrate, and a detector can bepositioned adjacent the free surface of the coating (i.e., the coatingsurface opposite the substrate) to measure the fluorescence intensityfrom which the thickness of the coating can be determined.

In another manifestation of the invention, fluorescence analysis is usedto determine any of a number of coating or film properties such as thedegree of cure, hardness, abrasion resistance, resistance to stain,tacticity, elasticity, molecular orientation, tensile strength,viscosity, modulus, stress state, wetness, resistance to smear orsmudge, and others by relating the change in the fluorescence of a probein the coating or film to a change in one or more of these properties.These properties may be measured individually, in combination, or inconjunction with the measurement of the coating thickness as previouslydescribed.

The ultimate properties of the polymeric network which constitutes amass such as a coating or film depend on various factors, such asmonomer structure, forms of initiation, reaction conditions and the rateand degree of polymerization. These properties affect themicroenvironment of the fluorescence probe and, thus, cause changes inits fluorescence. Probe fluorescence changes accompanying polymerizationin photochemical processes, and others, are related both to changes inthe microviscosity and the local polarity of the probe induced both bythe solid polymers and the resulting polymeric solutions. By measuringthe intensity of the fluorescence at two or more wavelengths andcalculating the ratio of the intensities (I.sub.λa /I.sub.λb) fordifferent predetermined coating or film-forming conditions such asdegree of cure, hardness, tacticity, elasticity, etc., calibrationcurves can be constructed from which dynamic properties can bedetermined using fluorescence measurements. For example, by using suchtechniques as infrared transmission in thin films (FTIR), differentialscanning calorimetry and the like, the degree of cure, as indicated bydouble bond conversion, can be determined. By correlating the measuredproperty with fluorescence intensity ratios at two wavelengths or byrecording the entire spectrum, for a given probe in a given coatingcomposition, calibration curves can be established such that in anindustrial setting, measurement of the intensity ratios can be used tomonitor the degree of cure of the coating or film for on-line oroff-line process control. In an analogous manner, calibration curves canbe established relating fluorescence intensity ratios to other coatingproperties. Based upon such calibration data, the foregoing method canbe used to monitor coating properties.

One method in accordance with the invention comprises the steps of:

providing a fluorescence probe in a curable polymeric mass such as acoating or film;

causing said fluorescence probe to fluoresce wherein said fluorescenceprobe emits radiation at a multiplicity of wavelengths of measurableintensities;

calculating the ratio of the intensity of the emitted radiation of theprobe at two or more predetermined wavelengths;

determining the monitored property of said mass based upon said ratio.The determination is made by comparing the observed ratios tocalibration data in which the ratios are correlated with an externalmeasure of the property. This method can be used in combination with themethod for determining the coating thickness if the fluorescenceemission of the substrate in the first method is distinct from thefluorescence emission of the probe.

The fluorescence probe technology can be used for on-line as well asoff-line analysis in both quality control and processing optimization.An on-line monitoring of the degree of cure or any of the otherproperties previously mentioned can be achieved by continuously orperiodically examining the fluorescence emissions spectra of the probewhich was doped into the monomer resin at a low concentration (typically0.02-0.01 wt. %) before the photocuring process. However, in some casesan off-line analysis is recognized to be more practical and economical.An off-line analysis can be achieved by applying a probe solution to thesurface of a coating or film or immersing the coating or film in thesolution such that the solution swells the polymer and the probemigrates or diffuses into the polymer coating or film. It has been foundthat polymer properties can be monitored regardless of whether the probeis pre-mixed with the coating polymer or diffused into the coating. Thedegree of cure of the polymer coating or film can be determined bymeasuring the fluorescence emission spectrum of the probe. The degree towhich the coating or film is swelled by the probe solution and the probeis absorbed by the coating is inversely proportional to the degree towhich it is cured.

Accordingly, another method in accordance with the invention comprisesthe steps of:

diffusing or impregnating a fluorescence probe into a cured polymericmass such as a coating or film;

causing said fluorescence probe to fluoresce wherein said fluorescenceprobe emits radiation at a multiplicity of wavelengths of measurableintensities;

calculating the ratio of the intensity of the emitted radiation of theprobe at two or more predetermined wavelengths;

determining the monitored property of said coating or film based uponsaid ratio.

Various fluorophores can be used as probes. These may be added to the tobe cured system (so-called extrinsic probes) or they may be intrinsic tothe system. Extrinsic probes may be chosen from various kinds includinga fluorescence probe which produces an excimer which has a fluorescenceemission distinctly different from the fluorescence emission of themonomer of said fluorescence probe, a molecule which produces afluorescence which is easily susceptible to quenching, or molecules withmultiple fluorescence emissions which depend on molecular conformationsin the excited state such as those exhibiting Twisted IntramolecularCharge Transfer (TICT) properties.

An apparatus useful in monitoring the aforementioned coating propertiesusing the fluorimetric methods of the invention is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows changes in the fluorescence spectra of dansylamide (DA)observed during thermally initiated polymerization of the TMPTA-VPmixture. Numbers above the fluorescence curves indicate the degree ofcure.

FIG. 2 illustrates a calibration curve for determining the thickness ofa silicone acrylate coating on a flat polyethylene terephthalatesurface.

FIG. 3 illustrates a calibration curve for determining the thickness ofa silicon acrylate coating on the shoulder of a PET bottle.

FIG. 4 illustrates a calibration curve for determining the thickness ofa silicone acrylate coating on the side wall of a PET bottle.

FIG. 5 illustrates the shift in the emission spectrum of 1,5-DASB in aurethane acrylate mixture as a function of degree of cure.

FIG. 6 illustrates a shift in the emission spectrum of 1,5-DASB in apolyester acrylate mixture as a function of degree of cure.

FIG. 7 illustrates the effect of coating thickness on the nascentfluorescence of a PET substrate.

FIG. 8 and FIG. 9 are calibration curves for determining the degree ofcure of a polymeric film using post cure diffusion of the probe for ashort (FIG. 8) and long (FIG. 9) soaking time.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that the thickness of cured polymeric coatingsand films and the degree of cure and other properties of polymericmasses such as coatings and films can be accurately and quickly measuredusing fluorescence methodology.

Often solid polymers are made in continuous processes from liquids whichcan be formed, shaped, delivered, or in some other way manipulated inthe liquid, and then dried or cured to form the solid, functionalproduct. This process of conversion from a mobile phase to an immobilephase is herein defined as curing. Cured polymeric coatings are used ina variety of applications to provide a durable surface withoutdetracting from the object itself. For example, coatings such assilicone acrylate may be applied to the outside surface of plasticbottles used to contain soft drinks. These bottles are typically madefrom polyethylene terephthalate (PET) and the silicone acrylate coating,when cured, provides a hard surface having long term durability enablingthe bottles to be reused. For environmental reasons, the ability toreuse such plastic bottles is highly desirable.

In order to coat plastic bottles or any substrate, it is necessary, notonly to have a method for coating the particular substrate in the mobileuncured phase and curing it to an immobile phase (be that solid, liquid,or gel) but, in many cases, it is also necessary to be able toaccurately and quickly determine the thickness of the coating and/or thedegree of cure of the coating and other properties as a measure of itsefficiency and effectiveness.

In measuring the thickness or coat weight of the coating on a substrateexhibiting nascent or doped fluorescence in accordance with theinvention, the uncoated support is irradiated to ascertain the intensityof the fluorescence emission at a given wavelength. The support is thencoated with the desired coating and the intensity of the fluorescenceemission of the support is remeasured. In all cases the coating or anagent in the coating must absorb the fluorescence of the substrate.Irradiation of the substrate through the coating has the effect ofproducing a light emission at the interface of the substrate and thecoating, which can be used to monitor the thickness of the coating in anindustrial setting. The thickness of the coating can then be obtainedfrom a coordinate calibration curve based upon the ratio of theintensities for the uncoated substrates and substrates coated withcoatings of known thicknesses.

In measuring the thickness or coat weight of a coating, fluctuations ininitial light intensity (I_(O)) and variations in the conversion factorof the detector of the fluorescence emission (D) with intensity canresult in significant error. This error can be corrected byincorporating a probe into the coating and monitoring the totalfluorescence emission from both the substrate and the probe at twoindividual wavelengths and then taking the ratio of two signalintensities. The coating thickness can be expressed by the followingequation: ##EQU1## where A,B, and C are known parameters and R is theratio of the emission intensity at two wavelengths. The unknownparameters can be found by a calibration procedure. If the signalintensity ratios R₁, R₂, and R₃, of three individual samples with knownthicknesses L₁, L₂, and L₃ are obtained experimentally, the parametersA, B and C can be calculated. Once the parameters are known, thethickness of the coating layer can be determined using the foregoingequation. The method is illustrated in Example 13.

The method of the present invention is useful in measuring coatingthicknesses up to about 1 cm. The upper limit will vary depending uponthe extent to which the coating absorbs the excitation radiation. Thereis no lower limit on the thickness of the coating.

Any of a variety of substrates may be coated and the coating thicknessmonitored in accordance with the present invention including mostnatural and synthetic polymeric films such as polyethylene,polypropylene, polystyrene, PET, polyurethanes, epoxies, vinyl such asPVC, polyamides and nylons, polyimides, elastomeric materials andrubbers, composites of materials, fiberglass as well as other materialssuch as wood, particle board, glass, cement, ceramics, metals, cotton,wool, synthetic fibers, paper, other cellulosics, and virtually anystructural material. Most materials, and most polymers exhibit a nascentfluorescence and any substrate having nascent fluorescence can be coatedand the thickness of the coating monitored as taught herein by causingfluorescence in that substrate. In the event that the substrate does notexhibit nascent fluorescence, the substrate may be doped with afluorescer to induce florescence. Fluorescers are commercially availablewhich can be used when necessary in this embodiment of the invention.The amount of the fluorescer used in the substrate can easily bedetermined such that a fluorescence of measurable intensity is obtainedtaking into consideration the absorption of the coating of bothexcitation and emission radiation.

Any of a variety of polymers can be monitored for thickness, degree ofcure and other properties in accordance with the teachings of thepresent invention provided that they transmit a sufficient amount of theexcitation energy to induce fluorescence in the substrate in the case ofthickness measurement and in the coating itself in the case of thedetermination of other properties. Examples of polymers includepolymerizable and heat cured or dried polymers. Specific examplesinclude any curable (e.g., radiation curable and thermally curable)acrylate such as silicon acrylates, urethane acrylates, epoxy acrylates,polyester acrylates such as trimethylolpropane triacrylate (TMPTA),urethane acrylate, ethoxylated trimethylolpropane triacrylate,1,1,1-trimethylolpropane triacrylate, dipentaerythritol pentaacrylate,pentaerythritol triacrylate, 1,1,1-trimethylolpropane trimethacrylate,1-vinyl-2-pyrrolidinone, and mixtures thereof as well as vinyl compoundssuch as styrene, PVC, and vinyl ethers and unsaturated polyesters. Alsoincluded are cycloaliphatic epoxides, aryl, aliphatic or aromaticepoxides such as bisphenol-A epoxide and diglycidyl ether of butane(DGEB) materials used for liquid crystals, holograms and the like.

In order to determine the thickness of a coating, the coating mustabsorb the fluorescence emission produced in the substrate. While almostall coating compositions will absorb the fluorescence to some degree, inmany cases it may be desirable to place an absorber in the coating forthis purpose. Any compound which partially absorbs the fluorescencewould be potentially useful. Complete absorption would preclude anythickness measurement. Those skilled in the art familiar with theapplication of Beer's law will understand the nature and amount ofabsorber that may be used. The fluorescence probes, described herein aregenerally also useful as absorbers in the measurement of coatingthickness. Hence, they provide a means to measure both thickness andother coating properties such as the degree of cure.

Among the myriad of industrial applications in which fluorescencemethodologies of the present invention are useful in monitoring coatingproperties such as thickness and/or degree of cure, several areparticularly notable: coating plastic beverage containers with ascratch-resistant finish, as previously mentioned, optical fibercoatings, coating floor tiles and flooring with an abrasion-resistant(no-wax) finish such as an acrylic or polyurethane-acrylates finish,coating papers and films with release films or finishes to providerelease sheets useful in the tape and label industries, and acrylatecoating of fiber glass.

In measuring the degree of cure of a polymeric mass such as a coating orfilm material, a fluorescence probe is incorporated into a curablepolymeric composition. The fluorescence probe is generally added to thecoating material in the amount of about 0.001 to 0.1% by weight of thecoating material. This methodology can also be used to monitor theproperties of multiple coatings on one substrate.

In order to effectively utilize the fluorescence information obtained,the fluorescence probe should absorb light at one wavelength and emitlight at a significantly different wavelength to prevent interference.In addition, in monitoring cure or other properties of the mass orcoating, the probe must experience a fluorescence shift as a result ofchemical and associated physical changes in the mass or coating.Preferably, the probe exhibits a multiplicity of emissions with a largeStokes shift. These emissions exhibit both a wavelength change and anintensity change. One example of a useful spectral shift is shown inFIG. 5 where the emission spectra of the probe 1,5-DASB are provided fora cured and uncured urethane acrylate composition (Example 8).

The fluorescent shift can be the product of a number of differentmechanisms. Certain fluorescence probes form intermolecular excimerswhich are excited dimers thought to be formed by a diffusion--controlledreaction between a molecule in its excited state and another molecule inthe ground state. A*+A→A+A+hν. Such excimers have a different emissionspectrum than the monomer, the spectrum is shifted to the shorterwavelengths for the monomeric emitter and to longer wavelength for theexcimer. Excimer emission has been shown to be a function of coatingviscosity. See Paczkowski and Neckers, supra.

Other fluorescence probes are capable of emitting radiation fromdifferent conformations, e.g., from twisted intramolecular chargetransfer (TICT) states. The shorter wavelength is thought to be due to acoplanar excited state conformation and the longer wavelength from anexcited molecule with a perpendicular conformation. This technique isbased upon the difference in fluorescence intensity from the paralleland perpendicular conformations of the excited state of the complex andis based on the dependence of the relative population of eachconformation on the microviscosity of the system. As the curing reactionproceeds, the steady state fluorescence emission spectra of the probeshave been found to exhibit hypsochromic spectral shifts due to theincrease in matrix microviscosity. A linear correlation between thefluorescence intensity ratio (R=Ipar/Iper) where Ipar and Ipercorrespond to the emission intensity values of the parallel andperpendicular conformations of the excited state respectively) and theextent of polymerization, measured by transmission FTIR spectrometry,has been obtained for different types of acrylated polymers cured withUV or visible (VIS) initiators.

The fluorescence probes useful in the invention include dansylamide,1-(N,N-dimethylamino)-5-n-butylsulfonamido-naphthalene (1,5-DASB),2-(N,N-dimethylamino)-5-n-butyl-sulfonamide-naphthalene (2,5-DASB),2-N,N-dimethylamino-6-propanoylnaphthalene (PRODAN),4-(N,N-dimethylamino) benzonitrile, (DMABN), ethyl 4-(N,N-dimethylaminobenzoate, 2-([4-(N,N-dimethylamino) benzoyl]oxy) ethyl methacrylate,butyl 4-(N-piperidino) benzoate, butyl 4-(N-morpholino) benzoate, butyl4-(N,N-dimethylamino) benzoate, n-butyl 2-dimethylamino-5-naphthalenesulfonate, N-(4'-cyanophenyl)-carbazole, N-(4'-butyl benzoate)carbazole,N-(1'-naphthyl)-carbazole, 9,9'-dianthryl, 4-dimethylamino-4'nitrostilbene, ethyl-4-dimethylaminobenzoate,2-dimethylaminonaphthalen-6-sulfonyl-N-butylamide,6-p-toluidinylnapthalene-2-sulfonyl-N-butylamide,6-(N-methylanilino)naphthalene-2-sulfonyl-N-butylamide,5-dimethylaminonaphthalen-1-sulfonyl azidridine, and all of thecompounds labeled 15-56 in the paper by W. A. Rettig (Angew. Chem. Int.Ed. English, 25 (1986) page 976) are included. Preferably thefluorescence probes are dansylamide (DA) and (1,5-DASB) and mostpreferably the fluorescence probe is (1,5-DASB). Fluorescence probeswhich exhibit TICT include 4-(N,N-di-methylamino)benzonitriles (DMABN)and 4-(N,N-dimethylamino)benzoates (DMB) among others.

The following methods can be used to measure the degree of cure of twocoatings. Commercially this methodology can be used to monitor thedegree of cure of the primary and secondary coating on an optical fiber.In accordance with one embodiment of the invention, a first fluorescentprobe (probe 1) is added to the primary coating, i.e., the first coatingapplied to the substrate and a second probe (probe 2) is added to thesecondary coating. If probe 2 absorbs at wavelengths at which probe 1 istransparent, for example, if probe 2 absorbs at longer wavelengths thanprobe 1, probe 2 can be excited at a wavelength (the excitationwavelength of probe 2) at which probe 1 is not excited. In this manner,the degree of cure in the secondary coating can be determined withoutinterference from probe 1 and the primary coating. The degree of cure ofthe primary coating can be determined prior to laying down the secondcoating by exciting probe 1 at its excitation wavelength, measuring theintensities of the emission of probe 1 at two wavelengths andcalculating the ratio of the intensities at the two emission wavelengthsand relating this to the degree of cure of the primary coating aspreviously described herein. Similarly, the degree of cure of thesecondary coating can be determined by exciting probe 2 at itsexcitation wavelength and measuring the emission intensities of probe 2at two wavelengths, calculating the ratio of the emission intensitiesand relating the ratio to the degree of cure as discussed herein.

In another embodiment, probe 1 absorbs and emits at longer wavelengthsthan probe 2. At short wavelengths, the extinction coefficient of probe2 is considerably higher than that of probe 1. Thus, at shortwavelengths, probe 2 can be excited without significantly excitingprobe 1. At longer wavelengths the extinction coefficient of probe 2 islow (e.g., zero) while the extinction coefficient of probe 1 is high. Atlong wavelengths, probe 1 can be excited without exciting probe 2. Thismeans that probe 1 can be excited specifically at a first wavelength atwhich probe 2 is not excited and the degree of cure of the primary layerin which probe 1 is present can be determined without interference fromprobe 2 by ratioing the emission intensities at two wavelengths, asdiscussed above, and relating the ratio to the degree of cure. Lightemitted by probe 1 is not absorbed by probe 2. Probe 2 can be excited ata wavelength at which probe 1 is substantially not excited. If theemission spectrum of probe 2 does not significantly overlap that ofprobe 1, the degree of cure of the secondary layer in which probe 2 ispresent, can be determined by ratioing the emission intensities andrelating this to the degree of cure as described herein.

In a third embodiment of the invention, the probes can be excitedsimultaneously. In this embodiment, probe 1 emits at a longer wavelengththan probe 2 but probe 1 and probe 2 have comparable extinctioncoefficient at the excitation wavelength. The emission of probe 1 is notabsorbed by probe 2. The radiation at that excitation wavelength excitesboth probe 1 and probe 2. If the overlap of the emission spectra of bothprobes is negligible, ratioing two emission wavelengths for probe 1 canbe used to determine the degree of cure of the primary coating andratioing the intensities of two emission wavelengths for probe 2 can beused to determine the degree of cure of the secondary coating. Due tothe fact that the emission spectra of probe 1 and probe 2 do notoverlap, the degree of cure of both the primary and the secondarycoating can be determined independently and simultaneously.

To add the probe to the coating or film "off-line", i.e., after applyingand curing the coating or film, the probe can be dissolved in a solventand the film can be immersed in the solution of the probe or thesolution of probe can be coated on the coating or film such that thesolution swells the polymer and the probe diffuses into the coating orfilm. The selection of the solvent will depend on the probe and thepolymer. Acetone and ethyl acetate are two examples. The concentrationof the probe can vary. Concentration on the order of 0.02% are usefulfor many applications. Concentrations of 0.03 to 0.12% are generallyuseful. To prevent any surface coating of the probe from interferingwith the analysis, the coating or film surface is rinsed with thesolvent. The coating or film is then allowed to dry and the coating isanalyzed in the same manner as if the probe had been directlyincorporated into the films. The probe is preferably dissolved in asolvent which swells the coating or film. The method is described inmore detail in Example 14.

The present invention also provides apparatus for practicing theforegoing methods. As with the methods, the apparatus may be designedfor off-line or on-line use. The apparatus includes a source ofexcitation energy, an analytical head which will preferably accessmultiple sites on a coated substrate of any size and shape, and a dataprocessing program so that the monitored coating property may bedetermined at any point along the substrate by comparison to calibrationdata. The detected radiation wavelength will vary with the nature of thesupport or the selection of the probe. Any fluorescent wavelength(200-800 nm) of measurable intensity is useful. The apparatus may bedesigned to detect selected wavelengths in the emission spectrum or todetect the entire spectrum. The apparatus may optionally include analarm circuit which is set to generate a signal if the comparison withthe calibration data indicates that there is an unacceptable variance inthe monitored coating property. Alternatively, the alarm circuit caninterface with other controls to adjust any of the coating conditionspreviously mentioned so as to directly effect a correction of thevariance.

Any convenient source of energy which will activate the fluorescenceemission may be employed and any means capable of detecting thefluorescence emission can be used in the present invention. Suitableexamples of excitation sources include ultraviolet radiation, electronbeam radiation, particle beam radiation, visible light, lasers (e.g., anargon laser), etc. Where light is used as the excitation source, thebandwidth of light which may be used may range from about 0.5 to 10 nm.This will depend on how well the excitation and emission wavelengths areseparated from one another as well as on various interferences fromother components. As the separation in nanometers increases, broaderbandwidth monochromatic radiation may be used.

The apparatus may employ a bifurcated optical fiber array wherein oneset of fibers provides the excitation energy to the substrate or coatingand another associated set of fibers is coupled to a photodetector anddetects fluorescence. This array may assume any design configurationnecessary to accommodate the substrate being coated. However, the arraywill likely be housed in a light-tight chamber. In one embodiment alinear array of optical fiber pairs (i.e., one fiber for excitation andthe other for detection) may be positioned immediately adjacent thecoated substrate for on-line monitoring of the coating.

The apparatus may include other components of a conventional fluorimetersuch as an electronic shutter, a monochromator, a photomultiplier tubeas the radiation detector, voltage to current converters which interfacewith the photomultiplier tube or CCD, focusing lenses, interferencefilters, neutral density filters, etc. An example of a conventionalfluorimeter is provided in Paczkowski, supra, Molecules, vol. 25, No. 2,1992 at page 552.

While the following examples will reference the measurement of degree ofcure, those skilled in the art will appreciate that any of the aforesaidproperties can be calibrated and measured in an analogous manner. Thedegree of cure of a coating can be determined by the fluorescence shiftexhibited in a cured coating. The degree of cure can be obtained from acoordinate calibration curve as shown in the examples below.

Example 1

Three flat polyethylene terephthalate samples coated with siliconeacrylate at various known thicknesses were individually irradiated withradiation of a wavelength of 302 nm using a SPEX Fluorolog IIfluorimeter (Xenon source) and a SPEX 1608 0.22 m monochromator. Thenascent fluorescence of the substrate was activated by the radiation andthe intensity of the nascent fluorescence emission through the curedcoating was measured at a wavelength of 380 nm with a SPEX Fluorolog1680 0.22 m monochromator and a PMT detector (slit conditions 1/1/1/1mm, scan speed 2 nm/sec.

Ratios of intensity of the nascent fluorescence emission through thecured coated to the intensity of the nascent fluorescence emission ofthe uncoated substrate, were calculated and these ratios, when plottedversus the previously determined thickness of the respective coating onan x,y coordinate graph, provides a linear calibration curve asillustrated in FIG. 2 from which coatings of unknown thickness on a flatsubstrate can be determined. The effect of coating thickness on thespectrum of nascent PET emission is shown in FIG. 7.

Example 2

The procedure of Example 1 was repeated except that the samples testedwere taken from the shoulder section of nine polyethylene terephthalatesoda bottles coated with silicone acrylate and cured. The coatingscontained 0.16% 1(N,N-dimethylamino)-5-n-butylsulfonamide naphthalene(1,5-DASB) as a fluorescence probe. The coatings were of variousthicknesses and were previously determined by independent means. A graphof the nine samples is shown in FIG. 3.

Example 3

The procedure of Example 2 was repeated except that the samples weretaken from side wall sections of six bottles. A graph of the six samplesis shown in FIG. 4.

Example 4

Secondary Optical Fiber Coating; 1,5-DASB probe was doped into themonomer solution (fiber optic secondary coating) at a concentration of=0.016 wt/%. A few drops of that monomer solution was squeezed into twoNaCl plates with a 15μ thick TEFLON (polytetrafluoroethylene) spacer atthe edges to specifically control the thickness of the film. Thefluorescence emission spectrum of the probe was acquired of samplescured for different periods of time with a medium pressure Hg arc lamp.The double bond conversion was measured using a Mattson Galaxy 6020infrared spectrometer and quantitated according to the disappearance ofabsorbance at 810 cm⁻¹. The results obtained are demonstrated in Table 1which shows the linear correlation y=1.38+2.63 x) between the C═Cconversion and the fluorescence ratio, I₄₇₀ /I₅₆₀. The latter wasmeasured with a Spex Model 2 Fluorimeter using an excitation wavelengthof 380 nm with slits of 0.2/0.2/1/1 mm.

                  TABLE 1                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             1.32                                                         0.14             1.80                                                         0.44             2.55                                                         0.49             2.75                                                         0.53             2.74                                                         0.56             2.80                                                         0.58             2.85                                                         ______________________________________                                    

Example 5

Primary Optical Fiber Coating; 1,5-DASB probe was doped into the monomersolution (fiber optic primary coating) at a concentration of =0.016wt/%. A few drops of that monomer solution was squeezed into two NaClplates with a 15μ thick TEFLON spacer at the edges to specificallycontrol the thickness of the film. The fluorescence emission spectrum ofthe probe was acquired of samples cured for different periods of timewith a medium pressure Hg arc lamp. The double bond conversion wasmeasured using a Mattson Galaxy 6020 infrared spectrometer andquantitated according to the disappearance of absorbance at 810 cm⁻¹.The results obtained are demonstrated in Table 2 which shows the linearcorrelation (y=1.63+2.32 x) between the C═C conversion and thefluorescence ratio, I₄₇₀ /I₅₆₀. The latter was measured with a SpexFluorolog Model 2 Fluorimeter using an excitation wavelength of 380 nmwith slits of 0.2/0.2/1/1 mm.

                  TABLE 2                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             1.65                                                         0.40             2.55                                                         0.88             3.70                                                         0.96             3.85                                                         ______________________________________                                    

Example 6

A solution of trimethylolpropane triacrylate (TMPTA), dipentaerythritolhydroxy pentacrylate (DPHPA), polyethylene glycol acrylate (PEGA)(40:40:20) was made homogeneous in a sonicator. 2,4-Diiodo-6-butoxyfluorone (DIBF) (5×10⁻⁴ M) and N-phenyl glycine (NPG) (5×10⁻² M) wasdissolved in the monomer by sonication. 1,5-DASB probe was doped intothe monomer solution at a concentration of ˜0.016 wt/%. The sample wasirradiated as in example 4) and 5). The double bond conversion wasmeasured using a Mattson Galaxy 6020 infrared spectrometer andquantitated according to the disappearance of absorbance at 810 cm⁻¹.The results obtained are demonstrated in Table 3 which shows the linearcorrelation (y=1.12+3.39 x) between the C═C conversion and thefluorescence ratio, I₄₇₀ /I₅₆₀. The latter was measured with a SpexFluorolog Model 2 using an excitation wavelength of 380 nm with slits of0.2/0.2/1/1 mm.

                  TABLE 3                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             1.10                                                         0.175            1.70                                                         0.225            1.90                                                         0.26             2.00                                                         0.275            2.07                                                         0.30             2.15                                                         0.325            2.20                                                         0.34             2.25                                                         ______________________________________                                    

Example 7

Primary Optical fiber Coating; PRODAN probe was doped into the monomersolution (fiber optic primary coating) at a concentration of 5×10⁻⁴ M. Afew drops of that monomer solution was squeezed into two NaCl plateswith a 15μ thick TEFLON spacer at the edges to specifically control thethickness of the film. The fluorescence emission spectrum of the probewas acquired of samples cured for different periods of time with amedium pressure Hg arc lamp. The double bond conversion was measuredusing a Mattson Galaxy 6020 infrared spectrometer and quantitatedaccording to the disappearance of absorbance at 810 cm⁻¹. The resultsobtained are demonstrated in Table 4 which shows the linear correlation(y=1.18+2.89 x) between the % C═C conversion and the fluorescence ratio,I₄₇₀ /I₅₆₀. The latter was measured with a Spex Fluorolog Model 2Fluorimeter using an excitation wavelength of 380 nm with slits of0.2/0.2/1/1 mm.

                  TABLE 4                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             1.20                                                         0.36             2.00                                                         0.525            2.70                                                         0.75             3.40                                                         0.925            3.85                                                         ______________________________________                                    

Example 8

PET Coating; 1,5-DASB probe was doped into the monomer solution(silicone acrylate coating from GE) containing an UV initiator at aconcentration of ˜0.016 wt/%. A few drops of that monomer solution wassqueezed into two NaCl plates with a 15μ thick TEFLON spacer at theedges to specifically control the thickness of the film. Thefluorescence emission spectrum of the probe was acquired of samplescured for different periods of time with a medium pressure Hg arc lamp.The double bond conversion was measured using a Mattson Galaxy 6020infrared spectrometer and quantitated according to the disappearance ofabsorbance at 810 cm⁻¹. The results obtained are demonstrated in FIG. 5which shows the fluorescence emission spectra for 1,5-DASB in theuncured and the 95% cured composition, and in Table 5 which shows thelinear correlation (y=1.34+4.82 x) between the % C═C conversion and thefluorescence ratio, I₄₇₀ /I₅₆₀. The latter was measured with a SpexFluorolog Model 2 Fluorimeter using an excitation wavelength of 380 nmwith slits of 0.2/0.2/1/1 mm.

                  TABLE 5                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             1.40                                                         0.16             2.10                                                         0.40             3.10                                                         0.50             3.85                                                         0.55             3.90                                                         0.60             4.20                                                         0.66             4.85                                                         0.69             4.65                                                         ______________________________________                                    

Example 9

PET Coating on Bottles: The degree of polymerization was measured on aGE silicone acrylate coating on a PET bottle as prepared at Pepsi ColaInternational. The same procedure as used to measure the thickness(Example 2) using an excitation wavelength of 302 nm was employed. Thescanning speed of the fluorimeter was at a 90 nm step speed.

Example 10

Polyester Acrylate; 1,5-DASB probe was doped into the monomer solution(polyester acrylate coating containing a UV initiator) at aconcentration of ˜0.016 wt/%. A few drops of that monomer solution wassqueezed into two NaCl plates with a 15μ thick TEFLON spacer at theedges to specifically control the thickness of the film. Thefluorescence emission spectrum of the probe was acquired of samplescured for different periods of time with a medium pressure Hg arc lamp.The double bond conversion was measured using a Mattson Galaxy 6020infrared spectrometer and quantitated according to the disappearance ofabsorbance at 810 cm⁻¹. The results obtained are demonstrated in FIG. 6and in Table 6 which shows the linear correlation between the % C═Cconversion and the fluorescence ratio, I₄₇₀ /I₅₆₀. The latter wasmeasured with a Spex Fluorolog Model 2 Fluorimeter using an excitationwavelength of 380 nm with slits of 0.2/0.2/1/1 mm.

                  TABLE 6                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             1.30                                                         0.075            1.40                                                         0.10             1.65                                                         0.20             1.95                                                         0.30             2.20                                                         0.40             2.40                                                         0.50             2.60                                                         0.55             2.55                                                         0.65             2.75                                                         0.70             3.10                                                         0.75             3.15                                                         0.85             3.25                                                         0.90             3.15                                                         ______________________________________                                    

Example 11

E606-6 Unsaturated Polyester Coating: DIBF (0.1% wt), OPPI anddiisopropyl dimethylaniline (DIDMA) (molar ratio of 1/2/3) was dissolvedin the monomer by sonication. 1,5-DASB probe was doped into the monomersolution at a concentration of ˜0.016 wt/%. The sample was irradiatedwith a 75 w dental lamp. The extent of conversion was measured using aPerkin Elmer DSC-4 thermal analyzer. The results obtained aredemonstrated in Table 7 which shows the linear correlation between theextent of conversion and the fluorescence ratio, I₄₇₀ /I₅₆₀.

                  TABLE 7                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0.00             0.12                                                         0.60             1.70                                                         0.76             2.20                                                         ______________________________________                                    

Example 12

Measurements on the polyester acrylate resin with the ORIEL Spectrographwith a Charge Coupled Device (CCD) detector. DIBF (0.1% wt),octyloxyphenyl phenyliodoniumhexafluoroantimonate (OPPI) and DIDMA(molar ratio of 1/2/3) was dissolved in the monomer (Armstrong Duracoat4) by sonication. 1,5-DASB probe was doped into the monomer solution ata concentration of ˜0.016 wt/%. A few drops of that monomer solution wassqueezed into two NaCl plates with a 15μ thick TEFLON spacer at theedges to specifically control the thickness of the film. Thefluorescence emission spectrum of the probe was acquired of samplescured for different periods of time with a medium pressure Hg arc lamp.The double bond conversion was measured using a Mattson Galaxy 6020infrared spectrometer and quantitated according to the disappearance ofabsorbance at 810 cm⁻¹. The results obtained are demonstrated in Table 8which shows the linear correlation (g=-0.224+0.37 x) between the C═Cconversion and the fluorescence ratio, I₄₇₀ /I₅₆₀. The latter wasmeasured with an Oriel Spectrograph with a CCD detector with ameasurement time of 2.5 msec. The spectra for three differentconversions are shown in FIG. 6. A linear correlation curve between theemission peak position and the extent of double bond conversion was alsoobserved. Therefore, emission peak position can also be used to monitorthe degree of cure.

                  TABLE 8                                                         ______________________________________                                        C = C Conversion Intensity Ratio                                              ______________________________________                                        0                0.5                                                          0.33             1.55                                                         0.85             2.90                                                         ______________________________________                                    

Example 13

Two wavelengths, 432 nmn and 505 nm, were selected which represent themaximum fluorescence emission of a substrate and the probe DASDrespectively. The fluorescence intensity (R=I₄₃₂ /I₅₀₅) for several ofknown thickness samples was obtained experimentally. Each value was anaverage of 15 measurements which were taken at various positions on acoated substrate of a specified coating thickness. The parameters in theforegoing equation can be obtained from a calibration procedure. Threesamples are chosen at random and the A, B, and C values were calculatedas:

    A=2.06

    B=-0.608

    C=-0.1712

The foregoing Eq. 1 was rearranged as follows: ##EQU2## L vs. (1/(B*R-C)was plotted with above values of B and C to give a straight line. Thedata points on the line represent the average of all the data points.Three data points were taken from the line to obtain a new set ofparameters as follows:

    ______________________________________                                        A              =     1.958                                                    B              =     -0.5240                                                  C              =     -0.1496                                                  ______________________________________                                    

The procedure was repeated using the new parameters to get more accuratevalues as follows:

    A=2.066

    B=-0.8209

    C=-0.444

With the values determined, the thickness of the coating can bedetermined using Eq. 1 based on known parameters.

Example 14

It has been demonstrated that a fluorescence probe which is doped intothe resin before cure can be used to monitor polymerization processes.The correlation between fluorescence intensity ratio and degree of cureis linear. Therefore, degree of cure can be monitored or determined byexamining the fluorescence intensity ratio. Furthermore, the degree ofcure of a polymer can also be determined by examining the fluorescenceintensity ratio of a probe which is migrated into the polymer network asshown here.

Three fiber optic coating samples were supplied. The outer layercoatings of the samples range in degree of cure from 70 to 90% asmeasured with a reflective FTIR. The fiber optic was cut into 30 cmlengths. The middle section of the fiber optic was soaked in a solutionof acetone containing 6.0×10⁻⁴ M DASD(5-N',N-dimethylaminoaphthalene-1-sulfonyl) probe. After the fiber opticwas removed from the solution, it was rinsed with pure acetoneimmediately to remove the probe on the coating surface. Then the fiberwas exposure to air so that the solvent inside the coating evaporated.The fluorescence emission spectra of the solvent-free sample weremeasured by exciting at 380 nm.

The higher the degree of cure of a coating, the more difficult it is fora probe to migrate into the coating. The fluorescence emission intensityis proportional to the amount of probe migrated into the coating. Duringthe soaking process, the probe will migrate faster into a sample with alow degree of cure. The fluorescence intensity is inversely proportionalto the degree of cure. The soaking time is limited to avoid saturatingthe coating with the probe. The experiment results indicate that a lowintensity is observed for a high cured sample and a high intensity isobserved for a low cured sample. FIG. 8 shows the correlation betweenfluorescence intensity and degree of cure for the fiber optic sampleswhich were soaked in the probe solution for 10 minutes. Once thecorrelation is established, the degree of cure can be obtained byexamining the fluorescence intensity.

A correlation between fluorescence intensity ratio and degree of curewas also observed in the case that the fiber optic samples soaked for arelatively long period of time, e.g., 1 hour or longer. Since thefluorescence intensity ratio reflects the extent of twisting of theprobe in the polymer network, the higher degree of cure, the larger theintensity ratio value. However, in the soaking process the probemolecules will occupy the larger size pores in the polymer network firstand then occupy the smaller size pores. If the soaking time is tooshort, most probe molecules only occupy the larger pores. With anincrease in the soaking time the probe molecules migrate further intothe polymer network. Finally, the network of the samples is saturated bythe probe molecules. In this case the correlation between fluorescenceintensity ratio and degree of cure could still be observed. FIG. 9 showsthe correlation between fluorescence intensity ratio and degree of curefor the fiber optic samples which were soaked into the probe solutionfor 1 hour. It is clear that the correlation is linear. Therefore, thedegree of cure can be obtained by examining the fluorescence intensityratio.

Having described the invention in detail with the preferred embodimentsillustrated by the examples and elsewhere herein, it will be apparentthat modifications and variations are possible without departing fromthe scope of the invention defined in the appended claims.

What is claimed is:
 1. A method for monitoring a property of a polymericmass comprising the steps of:adding a fluorescence probe compoundcapable of fluorescing to a polymeric composition which has the abilityto undergo changes in microviscosity, the ratio of the intensity of thefluorescence emission of said compound at two wavelengths changing inresponse to said changes in said microviscosity of said composition;curing said composition; causing said compound to fluoresce; measuringthe fluorescence of said compound; calculating the ratio of theintensities of fluorescence emission of said compound at two or morewavelengths; relating said ratio to the monitored property of saidcomposition, the monitored property not being double bond conversion. 2.The method of claim 1 wherein said method is used to monitor a propertyselected from the group consisting of degree of cure, hardness, abrasionresistance, resistance to stain, tacticity, elasticity, molecularorientation, tensile strength, modulus, the stress state of a solid,wetness and dryness.
 3. The method of claim 2 wherein said polymericmass is a film or coating and said fluorescence is caused by irradiatingsaid coating with actinic radiation.
 4. The method of claim 3 whereinsaid coating is a crosslinkable or polymerizable compositionfluorescence probe.
 5. The method of claim 4 wherein said fluorescenceprobe is characterized in that the emission spectrum of said probeexhibits a multiplicity of emissions with a large Stokes shift, and saidemissions exhibit both a wavelength change and an intensity change uponcuring said composition.
 6. The method of claim 4 wherein said probeexhibits a first set of excited state equilibrium conformations in saidcomposition prior to curing and a second set of excited stateequilibrium conformations in said composition after curing, and saidfirst and second conformations exhibit distinctly different emissionspectra.
 7. The method of claim 6 wherein said probe exhibits multiplefluorescence emissions resulting from twisted intramolecular chargetransfer in said probe.
 8. The method of claim 4 wherein said probe isselected from the group consisting of dansylamide,1-(N,N-dimethylamino)-5-n-butylsulfonamido-naphthalene (1,5-DASB),2-(N,N-dimethylamino)-5-n-butyl-sulfonamide-naphthalene (2,5-DASB),N-N-dimethylamino-5-propionylnaphthalene, 4-(N-N-dimethylamino)benzonitrile, ethyl 4-(N,N-dimethylamino benzoate,2-([4-(N,N-dimethylamino) benzoyl]oxy) ethyl methacrylate, butyl4-(N-pipridino) benzoate, butyl 4-(N-morpholino) benzoate, butyl4-(N,N-dimethylamino) benzoate, n-butyl 2-dimethylamino-5-naphthalenesulfonate, N-(4'-cyanophenyl)-carbazole, N-(4'-butylbenzoate)-carbazole, N-(1'-naphthyl)-carbazole and 9, 9'-dianthryl. 9.The method of claim 8 wherein said fluorescence probe is 1,5-DASB. 10.The method of claim 3 wherein said coating includes a curable acrylateselected from the group consisting of silicon acrylates, urethaneacrylates, epoxy acrylates, polyester acrylates and unsaturatedpolyesters.
 11. The method of claim 3 wherein said coating includes avinyl compound selected from the group consisting of styrene, vinylchloride, and vinyl ethers.
 12. The method of claim 3 wherein saidcoating includes an epoxide.
 13. The method of claim 1 wherein the stepof relating includes comparing said ratio to calibration data in whichsaid monitored property is independently determined as a function ofsaid ratio.
 14. The method of claim 1 wherein one of said two or morewavelengths is about 200 nm to 400 nm.
 15. The method of claim 14wherein said one of said two or more wavelengths is 302 nm.